1. Field of the Invention
The present invention relates to analog optical transmission devices by an optical SCM method, and more specifcally to a device capable of suppressing noise or distortion which increases due to reflected light, and noise or distortion which increases due to reflected return light to a light source and unstableness of the light source. Further, it relates to an analog optical transmission system capable of reducing optical beat interference noise which causes a problem when optical signals from a plurality of light sources are received in totality.
2. Description of the Background Art
An optical SCM (Sub-Carrier Multiplexing) transmission method is a method of converting a frequency-modulated electrical signal to be transmitted into laser light which is intensity-modulated by the signal and then transmitting the light, not requiring A/D and D/A converters unlike digital transmission by on/off of light and further having a characteristic of an extremely broad band and low loss compared to the conventional transmission method using a coaxial cable. Therefore, in recent years, its practical use has been highly expected.
In this optical SCM transmission method, the following problems are known.
Firstly, when multiple reflected light occurs in an optical fiber, noise and distortion increase to deteriorate a transmission characteristic.
Secondly, when reflected light is coupled to a light source, the state of the light source becomes unstable to increase noise and distortion.
Thirdly, when optical signals outputted from a plurality of light sources are received in totality, if wavelengths of optical signals are close to each other, optical beat interference (OBI) noise occurs to deteriorate the transmission characteristic.
The optical signal is reflected in an end surface of an optical component, a connector end surface of the optical fiber, etc. Or, Rayleigh scattering in the optical fiber, etc., also causes reflected light.
Among the above three problems, as an example of a device of suppressing the first transmission characteristic deterioration, an analog optical transmission device and an optical fiber amplifier are disclosed in Japanese Patent Laying-Open No. 5-291671.
FIG. 17 is a block diagram showing the structure of the conventional analog optical transmission device disclosed in Japanese Patent Laying-Open No. 5-291671. The device in FIG. 17 includes a multiplex portion 501, an adding portion 502, a picture signal input terminal 503, amplifiers 504 and 508, a semiconductor laser device 505, an optical fiber 506, a photo-receptive device 507, a picture signal output terminal 509, and optical connectors 510 and 511.
Analog electrical signals of N channels (ch1 to N) are RF-multiplexed in the multiplex portion 501, further in the additional portion 502 a pilot signal is added thereto, and then inputted to the picture signal input terminal 503. The electrical signal inputted in the picture signal input terminal 503 is amplified in the amplifier 504 and then converted into an optical signal by the semiconductor laser device 505. The optical signal obtained by the semiconductor laser device 505 is transmitted through the optical fiber 506 and the optical connectors 510 and 511 to the receiving side. The transmitted optical signal is again converted into an electrical signal in the photo-receptive device 507, further amplified in the amplifier 508, and then outputted from the picture signal output terminal 509.
In the above operation, part of the optical signal outputted from the semiconductor laser device 505 is reflected in the optical connectors 510 and 511 or subjected to Rayleigh scattering in the optical fiber 506. Further, part of reflected light is re-reflected, causing multiple reflected light which proceeds in the same direction as that of the optical signal. Generally, in the semiconductor laser, since wavelength chirping exists associated with electrical-optical conversion, the reflected light which proceeds in the same direction interferes with the optical signal at the time of optical-electrical conversion to cause unfavorable electrical intensity modulation, resulting in interference noise. Therefore, as it is, it is expected that noise or distortion will occur in the electrical signal outputted from the picture signal output terminal 509. For this reason, the device in FIG. 17 adds a pilot signal to the electrical signal to be transmitted, converts the electrical signal into an optical signal and then transmits the optical signal, thereby dispersing the power of the interference noise over a wide frequency band. This reduces the power of the interference noise in the transmission frequency band, resulting in reduction in noise or distortion by reflected light.
Also disclosed in Japanese Patent Laying-Open No. 5-291671 there are conditions in which the frequency of the pilot signal is not more than the frequency corresponding to the spectrum line width of the semiconductor laser. It is expressed in FIG. 17 that adding a pilot signal which satisfies the conditions sufficiently reduces noise or distortion by reflected light which occurs in the optical fiber 506 and the optical connectors 510 and 511. However, in the device of FIG. 17, adding a pilot signal newly causes a second order intermodulation distortion (hereinafter referred to as IM2) between the RF-modulated analog electrical signal and the pilot signal.
Furthermore, a structure like that of Japanese Patent Laying-Open No. 5-291671 is used in U.S. Pat. No. 5,373,385. In this patent, the frequency of the additional signal is defined to be outside the band of the signal to be transmitted. Therefore, although the additional signal does not directly have an adverse effect on the signal to be transmitted, as is the same in Japanese Patent Laying-Open No. 5-291671, IM2 newly occurs to have an adverse effect on the signal to be transmitted.
On the other hand, U.S. Pat. No. 5,430,569 discloses a structure capable of reducing IM2 which newly occurs by an additional signal. In this patent, when IM2 occurs in the band allotted for transmission of the signal to be transmitted, a predistorter is used for reduction in its effect. When IM2 occurs only outside the band, the predistorter is omitted from the structure.
As a method of suppressing the second transmission characteristic deterioration, a method of inserting an optical isolator between a light source and an optical fiber so as not to couple reflected light to the light source is generally adapted.
As a method of suppressing the third transmission characteristic deterioration, Japanese Patent Laying-Open No. 6-177840 discloses an optical communications method of suppressing OBI noise. FIG. 18 is a block diagram showing the structure of the conventional optical transmission system using the optical transmission method described in Japanese Patent Laying-Open No. 6-177840. The system in FIG. 18 includes transmitting terminals 600 to 602, receiving terminals 603 and 604, optical fibers 605 and 606, and an optical star coupler 607. Each of the transmitting terminals 600 to 602 has oscillators 608.sub.1 to 608.sub.3, electrical modulators 609.sub.1 to 609.sub.3, and optical modulators 610.sub.1 to 610.sub.3. The receiving terminal 603 includes an optical demodulator 611, frequency selective filters 612.sub.1 to 612.sub.3, electrical demodulators 613.sub.1 to 613.sub.3, and oscillators 614.sub.1 to 614.sub.3. The receiving terminal 604 includes an optical demodulator 615, frequency selective filters 616.sub.1 to 616.sub.3, electrical demodulators 617.sub.1 to 617.sub.3, and oscillators 618.sub.1 to 618.sub.3.
Each of the oscillators 608.sub.1 to 608.sub.3 (f1 to 3 shown in the drawing), 614.sub.1 to 614.sub.3, and 618.sub.1 to 618.sub.3 outputs a sub-carrier with the frequency of f1 to f3 (electrical signal), respectively. Each of the electrical modulators 609.sub.1 to 609.sub.3 modulates the sub-carrier by the input signal. Each of the optical modulators 610.sub.1 to 610.sub.3 modulates a main carrier (an optical signal) with the wavelength of .lambda.1 to .lambda.3 by the sub-carrier, respectively. The optical star coupler 607 multiplexes the main carriers and divides the multiplexed carriers. Each of the optical demodulators 611 and 615 demodulates the main carrier. Each of the frequency selective filters 612.sub.1 to 612.sub.3 and 616.sub.1 to 616.sub.3 selects the sub-carrier with the frequency of f1 to f3 from the optical demodulator output, respectively. Each of the electrical demodulators 613.sub.1 to 613.sub.3 and 617.sub.1 to 617.sub.3 demodulates the sub-carrier.
Described below is operation of the system in FIG. 18.
When the oscillators 608.sub.1 to 608.sub.3 output the sub-carriers with the frequencies of f1 to f3, respectively, the electrical modulators 609.sub.1 to 609.sub.3 modulate the sub-carriers by the input signals (1) to (3). Next, the optical modulators 610.sub.1 to 610.sub.3 intensity-modulate the main carriers (optical signals) with the wavelengths of .lambda.1 to .lambda.3 by the modulated sub-carriers, respectively. The modulated main carriers are each transmitted through the optical fibers 605 to the optical star coupler 607, in which they are multiplexed and divided, and then transmitted through the optical fibers 606 to the receiving terminals 603 and 604. In the receiving terminals 603 and 604, each of the optical demodulators 611 and 615 respectively demodulates the transmitted main carriers, and the frequency selective filters 612.sub.1 to 612.sub.3 and 616.sub.1 to 616.sub.3 respectively select the sub-carriers with the frequencies of f1 to 3 from the each optical demodulator output. Then, when the electrical demodulators 613.sub.1 to 613.sub.3 and 617.sub.1 to 617.sub.3 demodulate the selected sub-carriers using the sub-carriers outputted from the oscillators 614.sub.1 to 614.sub.3 and 618.sub.1 to 618.sub.3, respectively, the input signals (1) to (3) can be obtained.
In the above operation, at the time of optical demodulation, OBI noise is caused when wavelengths of two main carriers are adjacent to each other. Therefore, the temperature or the bias current of the optical sources included in the optical modulators 610.sub.1 to 610.sub.3 is changed to periodically change the wavelengths .lambda.1 to .lambda.3 of the main carriers independently for each of the transmitting terminals 600 to 602. This can make the time when the frequency of OBI noise matches to the sub-carrier frequency extremely short, thereby decreasing the effect of the OBI noise. For example, in analog signal transmission for CATV, if the time when the frequency of OBI noise matches to the sub-carrier frequency is shortened in the above manner, the OBI noise hardly affects the receiving condition. Especially, when the modulation signal is an analog television signal in the cable television system, if the OBI noise effects on the analog signal momentarily, it cannot be recognized on a TV screen.
Further, disclosed in U.S. Pat. No. 5,532,865 is an optical communications method capable of suppressing beat interference using another method. The method is structured such that an inputted optical signal and another optical signal from an optical source are coupled and the obtained optical signal is once branched and one of the branched signal is then outputted. The other of the branched optical signals is converted into an electrical signal to examine whether the frequency of OBI noise is adjacent to the sub-carrier frequency, and if adjacent, the wavelength of the optical source is changed to change the frequency of OBI noise, thereby removing the effect of the OBI noise.
However, in each of the above shown conventional examples, the following problems still remain.
That is, for the first problem, in both Japanese Patent Laying-Open No. 5-291671 and U.S. Pat. No. 5,430,569, there is a possibility that IM2 may occur in the transmission band to deteriorate the transmission characteristic. Further, in U.S. Pat. No. 5,373,385, although IM2 in the transmission band can be reduced, a predistorter therefor is newly required. For the predistorter, it is required to accurately perform adjustment of distortion power for compensation and an amount of phases, and this adjusting operation is not easy.
For the second problem, in the conventional method, an expensive optical isolator is provided to increase the cost of the device. Additionally, the optical isolator is generally installed in a distributed feedback semiconductor laser (DFB-LD) module, and conventionally in most cases, for the device performing optical SCM transmission, this expensive DFB-LD has been adapted as the optical source. On the other hand, a Fabry-Perot-type semiconductor laser (FP-LD) module is generally used for digital optical communications, very low-priced but not containing an optical isolator. If the second transmission characteristic can be suppressed without an optical isolator, the price of the device can be made extremely low.
For the third problem, the method shown in Japanese Patent Laying-Open No. 6-177840 has a possibility of decreasing the effect of the OBI noise as far as a picture signal is transmitted. However, for example, when the modulation signal is a digital modulation signal, momentary effect of noise causes a burst error of the digital signal. In this case, even with error correction, there is a possibility that the burst error cannot be corrected sufficiently. In addition, also when a picture signal is transmitted, intermodulation distortion occurs between the sub-carrier changing the wavelength of the main carrier of the optical signal and the sub-carrier for transmitting the picture signal.
Further, in Japanese Patent Laying-Open No. 6-177840, the OBI noise cannot necessarily be reduced sufficiently. This is because the following reason: the reducing effect of the OBI noise is due to a chirping characteristic of the semiconductor laser, substantially depending on not only the frequency of the additional signal to be added to the semiconductor laser but also its optical modulation index and bias current value. Therefore, in order to sufficiently reduce the OBI noise, it is necessary to consider these parameters altogether. However, in Japanese Patent Laying-Open No. 6-177840, the amount of reduction in the OBI noise is not quantitatively evaluated as such, and therefore the OBI noise cannot be necessarily reduced sufficiently.
Furthermore, in U.S. Pat. No. 5,532,865, in order to change the wavelength of the optical source, for example, it is required to change the temperature or the bias current of the optical source. When the bias current is changed, the optical modulation index, which is an important parameter of the optical SCM transmission system, is also changed, thereby causing the danger of undesirably inviting deterioration of a distortion characteristic and deterioration of a C/N characteristic in addition to the OBI interference. On the other hand, in order to change temperature, it is necessary to install a part therefor. In addition to the description as to changing temperature, in general, the FP-LD module does not have a temperature control function, and when it is used as an optical source for the optical SCM transmission, a part for temperature control must be installed.